1. Field of Invention
This invention relates to the field of signal transit time sensors, in particular sensors based on ultrasonic transit times.
The rate of propagation of a signal within a medium depends on the composition of the medium. In this way, by measuring the rate of propagation, it is possible to draw a conclusion regarding the medium itself. The corresponding measurement methods are often based on the propagation of ultrasonic waves. The measurement object, which in the case of the fluid to be tested may be in the form of a fluid-carrying line, for example, is arranged inside a measurement zone, which separates an ultrasonic transmitter from an ultrasonic receiver. If the length of the measurement zone is known, then it is possible with the help of the signal transit time to determine the rate of propagation from the transmitter to the receiver. If only a relative change is of interest, then it is possible to deduce directly the relative change in the rate of propagation from the relative change in the signal transit time as long as the measurement zone does not change in an unknown manner.
The measurement zone may be divided into different regions. If a medium flows through a line, the measurement zone is composed of a first region, comprising the walls of the line, and a second region, through which the actual measurement medium passes. Then if the composition of the medium changes, the resulting change in the signal transit time can in general be attributed solely to the change in the signal transit time in the second region, because the signal transit time in the first region does not change given a suitable choice of walls. If the signal transit time in the first region is known through a knowledge of the material of which the wall is made and the dimensions thereof, it is also easy to determine the absolute signal transit time in the second region.
In a hemodialysis treatment, blood is removed continuously from a patient in an extracorporeal blood circulation, purified by a hemodialyzer and returned to the patient. As an artificial kidney treatment, simultaneous removal of the liquid that has not been separated is not necessary. In this case, a given quantity of liquid is usually withdrawn from the patient during the treatment. However, if the liquid is withdrawn too rapidly, there may be unwanted side effects such as hypotension or an excessive drop in blood pressure.
The reduction in blood volume caused by the removal of liquid leads to an increase in blood density, i.e., a decrease in the blood-water content, which is in general also manifested in an increase in hematocrit; therefore, monitoring of the extracorporeal blood with the help of an ultrasonic transit time sensor was proposed in U.S. Pat. No. 5,230,341. In German patent application DE 100 51 943 A1 the influence of blood density variations on pulse wave transit time measurements is treated, where in one embodiment ultrasonic transit time measurements are employed to determine the blood density.
2. Description of the Prior Art
Ultrasonic transit time measurements have been proposed in German patent application DE 198 09 945 A1 also for monitoring the composition of the dialysis liquid. The attenuation of ultrasonic signals is, as described for example in European patent application EP 0 899 564 A2, evaluated for the recognition of air bubbles during the infusion of liquids into a patient.
For control and analysis of such sensors, certain electronic circuits are used to ensure the required accuracy. An example of such a circuit is given in German Patent DE 34 20 794 C2.
U.S. Pat. No. 6,542,761 discloses a multifunctional ultrasonic transit time sensor for performing measurements on extracorporeal blood; with this sensor, an inexpensive disposable part may be used for the line that carries blood through the measurement zone.
In the field of sensors on natural gas pipelines European patent EP 0 902 883 B1 describes a method in which the zero crossings of a received sound signal are determined for propagation time measurements of a sound signal.